Recently, to reduce a rolling resistance of a tire and to improve a wet grip performance, it is proposed to compound silica into a tread rubber as a rubber reinforcement agent in place of carbon black. However, the silica is inferior in electrical conductivity and increases electric resistance of the tread rubber. Therefore, in a tire comprising a tread rubber containing silica, the static electricity is accumulated in the car, and it may possibly cause an electromagnetic wave obstacle such as radio noise.
The following Patent Document 1 proposes a tread rubber (a) shown in FIG. 11 (A), for example. This tread rubber (a) comprises a base rubber section (b) made of an electrically conductive rubber containing carbon, a cap rubber section (c) made of an electrically non-conductive rubber containing silica and superposed on the above-mentioned base rubber section (b) outwardly in the radial direction, and a conducting section (d) made of an electrically conductive rubber which extrudes from the base rubber section (b) outwardly in the radial direction, penetrates the cap rubber layer (c), and is exposed on an tread ground contact area. Such a tread rubber (a) can be formed in extrusion molding.
To suppress uneven wear occurring between the above-mentioned conducting section (d) and the cap rubber section (c), it is preferable to form a width W of the conducting section (d) as small as possible. When the width W is set to be small in the extrusion molding, the conducting section (d) may terminate inside, so that it may not possibly conduct. In that case, the termination occurred inside the conducting section (d) cannot be easily judged, so that it leads to a problem of need to perform a one hundred percent inspection of the tire conducting.
The following Patent Document 2 proposes, as shown in FIG. 11 (B), to apply so-called strip winding method that the rubber strip is helically wound in place of the above-mentioned extrusion molding so as to form the above-mentioned base rubber section (b), cap rubber section (c), and conducting section (d). This strip winding method has an advantage that the termination (bad conducting) of the conducting section (d) can be easily checked as a tension crack of the winding rubber strip.
However, the pneumatic tire preferably comprises the above-mentioned base rubber section (b) also made of the electrically non-conductive rubber containing silica in order to reduce more the rolling resistance. In that case, it is necessary to provide in the base rubber section (b) with the conduction section so as to penetrate into the base rubber section (b) and extending in the radial direction, and to connect by butting together the radial outer end of the conduction section and the radial inner end of the conduction section positioned in the above-mentioned cap rubber section (c). The conduction section is formed of a thin rubber strip having a thickness of about 2 mm. Therefore, it is difficult to butt the conduction sections with each other, and there is a problem of reducing notably the productive efficiency.
Incidentally, other than butting together the conduction sections, as shown in FIG. 11 (C) for example, it is proposed to form a single conducting section (d) penetrating both of the base rubber section (b) and the cap rubber section (c) by spirally winding a fine rubber strip (p) being thin in a strip width (wp) outwardly in the radial direction in such a way as to stack it up. In this case, the above-mentioned conducting section (d) divides the base rubber section (b) and the cap rubber section (c) into a right half and a left half. Therefore, it becomes necessary that the divided base rubber sectional parts b1 and b2, and the cap rubber sectional parts c1 and c2 are separately formed with the rubber strip; and the production efficiency decreases. Furthermore, between the conducting section (d) and the base rubber sectional parts b1 and b2, and between the conducting section (d) and the cap rubber sectional parts c1 and c2, an air pocket is liable to occur. It deteriorates the tire uniformity and causes a problem of elevation of the incidence of product defects. In view of loosing the winding shape of the rubber strip (p), namely the width (wp), a width W of the conducting section (d) cannot be too small. Therefore, it becomes difficult to sufficiently reduce the uneven wear caused by the conducting section (d).
The following Patent Document 3 discloses that the both ends of the rubber strip are butted with each other for conduction.    Patent Document 1: Japanese Laid-open Patent Publication No. H9-71112;    Patent Document 2: Japanese Laid-open Patent Publication No. 2006-137067:    Patent Document 3: Japanese Laid-open Patent Publication No. 2008-285070.